I love this stuff. Found out about it back in the 60s from a retired diesel mechanic. It's worked everytime I've used it. I usally add it to the oil about 300 miles before it's going to be changed. Well maintained engines don't "need " it but it doesn't hurt. Those that buy abused engines can sometimes benifit from it's property's if things aren't to bad. Stuck lifters and clogged passages get opened up with MMO.

The only downside I can see is MMO loosening up deposits & possibly scoring cyl walls & bearings with those deposits being pumped back into the oiling system. But if the engine is in poor shape anyway, why not give it a shot.

I started changing out the plugs first then the plug wires and yet the miss was still there every once in a while.

I then changed the vacuum lines thinking maybe the spark was jumping around but that was not it either.

I then pulled the distributor thinking the shaft might be shorting out from time to time but that also did not fix the problem.

We checked the battery and all kinds of stuff with no luck.

A good friend of mine who is in his late 70's told me to try one quart of Marvel Mystery Oil and when I asked him why he said I think you have a lifter that is handing up every once in while and that's just what the problem was.

Once I added the Marvel Mystery Oil I never had the problem again.

Crazy thing is it really felt like an electrical problem and it was not.

I always have Marvel and use it for many things. Using the right oil helps too in some cases.

Hydraulic lifters years ago could be a problem in a performance car. In the mid 60s while still working for a Ford dealer, customers with 390 GTs were having trouble with valve train float. Especially in hot weather. I would cure it by draining out the Rotunda 10 W 30 and put in 30 and sometimes 40 wt. DA oil. They thought I was a magician. Those engines picked up 1000 RPM. From floating between 4K and 4.5 K to 5,000 and 5500. These were new or fairly new factory stock Fords. Dealer was not happy with me using none Ford oil. That kind of thinking is one reason I opened my own shop in 1967.

I ran into that hydraulic lifter problem many times during the 60s and 70s with many brands of cars. One incident after I opened my own shop was a 3 year old 71 Coronet 426 Hemi. My customer had me check it out as it was running weird. It was very hot weather like now. Acted like a bad cam. I took valve cover off while engine was still hot. I opened a random valve and was going to roughly check real quick the lift on the cylds in question with a machinist rule to see if a lobe had wiped out. When I went to put the 6" rule to the retainer the valve stated moving toward closing. The damn 71 426 Hemis had hydraulic lifters!!! I had worked on 426 hemis before, but not a 71. I looked in my new Motors Manual at the valve settings---and sure enough for 71 426 hemi the valve adjustment spec read ZERO!

I called my customer and asked what oil was in it. It was a major brand 10 W 30. I drained the oil, changed the filter to a racing filter and put DA 50 wt. in. That cured the problem. In later years hydraulic lifters improved. And of course in the after market there are choices of various types of hydraulic lifters. Now we have the problem with oils with less phosphorous additives that created a disaster for flat tappet solid and flat surface hydraulic Lifters. The oil companies decided most modern engines had roller type lifters and backed off those important additives ness. for flat tappet cams. Prob. cheaper to make. But in a short time ---The American Ingenuity and business people saw an opportunity for a new lubrication market and rose to the problem.

Today there are countless brands of additive's and oils to accommodate flat tappet cams. Which I use with all my customers engines as most are flat solids or solid roller. I do a few hydraulic. The roller lifters , solid or hydraulic do not need the additives or the oil with them--the industry claims But all my engines get that addatve and the oil with it. Even rollers.

As for what oil I use? The last couple years I have been hung on Brad Penn. here why. I was brought a Boss 302 with 4,000 miles on the rebuild. When I tore it down I discovered the crank was .010 .010. The rod bearing were STANDARD mains were correct but the rod bearing clearance was .0125 and survived 4,000 miles--mostly cruises --. The bearing showed some scuffing--but still had some crush.

My customer had been using Brad Penn 20 W 50. The engine must have had some bottom end noise--but with solid lifters and loud Flowmasters it was not noticed. I guess this is a classic case of too loose is better than too tight. But 12.5 thousands is carrying that idea a little far. He did tell the oil pressure would drop below 20# at idle when hot. But stay at 50# hot at 2500 RPM.

Many of my peers also use Brad Penn. It has the right stuff.

I rebuilt the rods with new bolts and of course used .010 Clevites. The crank polished up with no problem with just a scotch brite belt.

Funny part is: His former build sheet showed .010 rod bearings. Now it does have them.